The present patent application claims the benefit of earlier Japanese Patent Application No. 11-363317 filed Dec. 21, 1999, the disclosure of which is entirely incorporated herein by reference.
1. Field of the Invention
The present invention relates to a probe pin for testing electric characteristics of an apparatus, such as a semiconductor integrated circuit, an LCD, a magnetic recording device, or the like, and to a method for fabricating the probe pin. The invention also relates to a probe card using a set of probe pins.
2. Description of the Related Art
A test probe or a probe card is generally used to test the electric characteristics of a semiconductor integrated circuit printed on a wafer before the wafer is cut into chips. As the pattern of a semiconductor integrated circuit becomes fine and dense, the pitch of the probe pins have to be reduced. A fine-pitch probe can be fabricated by, for example, forming a silicon single crystal pin by a VLS (vapor liquid solid) technique (R. S. Wangner and W. C. Ellis, Appl. Phys. Lett. 4, 1996 at 89). In this technique, a metal, for example, gold (Au) is placed on a substrate, and this metal is heated in the gas phase containing the composition of the probe (that is, silicon). Then, silicon is deposited in the solid phase via the molten metal in the alloy liquid phase. This method allows a silicon single crystal probe to be formed easily and accurately at a fine pitch making use of crystal growth. Because a single crystal silicon probe has a high electric resistance, the silicon probe is generally coated with a low resistance metal, such as gold.
FIG. 1 illustrates a conventional silicon probe 100. The silicon probe 100 has pin core 102a rising in the vertical direction, a silicon wiring layer 102b extending in the horizontal direction, a nickel-phosphorus seed layer 103, and a gold (Au) plating layer 104 covering the seed layer 103. The vertical pin core 102a, the seed layer 103, and the plating layer 104 form an individual test pin 105. On the other hand, the horizontal wising layer 102b, the seed layer 103, and the plating layer 104, form a lead electrode 106 for extracting an electrode from the test probe 105. The test pin 105 is connected to an external circuit (for example, a tester) via the lead electrode 106.
To fabricate the conventional test probe shown in FIG. 1, a wiring pattern corresponding to the lead electrode 106 must be formed prior to forming the single crystal silicon pin rising in the vertical direction. The wiring pattern can be formed by, for example, forming a silicon layer on a sapphire substrate 101, coating the sapphire substrate with a photoresist, patterning the photoresist into a horizontal wiring pattern, and etching the silicon layer into the lead using the photoresist as a mask.
However, the conventional silicon probe requires a certain space around each pin in order to extract and arrange the lead on the substrate. In addition, each lead extending from the associated pin must be arranged at a certain distance from the other leads, so that signals propagating through adjacent leads will not interfere each other. These factors greatly limit the freedom of producing two-dimensional layout of a probe, especially with respects to the positions and the density of the probe pins. This limitation is a fatal obstacle to producing a fine-pitch probe for testing a highly dense circuit.
Another problem in the conventional probe set is that if the leads from the associated probes are arranged in the two-dimensional manner, the wiring length becomes inevitably long, and in addition, the lengths of the leads extending from different probes differ from one another. Consequently, variation occurs in signal transfer among different leads when measuring electric characteristics at a high frequency. The long lead causes the contact resistance to increase between the probe pin and the electrode formed on the substrate, which results in a measurement error.
Therefore, it is an object of the invention to overcome the problems in the prior art technique, and to provide a probe pin for testing electric characteristics of an apparatus, such as a highly integrated semiconductor circuit.
It is another object of the invention to provide a probe assembly, which is a combination of a probe pin and an electrode connected to the base of the test probe. This probe assembly allows the pitch of probe pins to be reduced when a number of pins are arranged on a board, and the lengths of the leads from the probes can be made almost equal. The probe assembly has a low contact-resistance, and is suitable to measurement at a high frequency.
It is still another object of the invention to provide a probe card with a superior high-frequency characteristic, which is brought into contact with a high-dense semiconductor wafer to collectively test electric characteristics of the integrated circuits formed on the wafer.
It is still another object of the invention to provide a method for fabricating a probe pin for testing electric characteristics of, for example, a semiconductor circuit, and having a superior high-frequency characteristic.
To achieve the objects, a probe pin for testing electric characteristics of an apparatus comprises a silicon pin core, and a conductive layer covering the entire surface of the silicon pin core. In this text, xe2x80x9ccovering the entire surfacexe2x80x9d means that not only the tip or the side face of the silicon pin, which is brought into direct contact with an integrated circuit to be tested, but also the bottom face of the silicon pin core is coated with the conductive layer. By coating the bottom face of the silicon pin, the bottom face of the silicon pin is connected directly to the electrode placed in the print wiring board, which is further connected to a tester. The direct connection between the bottom face of the silicon pin and the electrode eliminates the necessity of leads extending from the side face of the associated silicon pins in a two-dimensional plane. Consequently, the density of the silicon pins is greatly increased when multiple pins are arranged on a wiring board.
The probe pin can be used to test or evaluate various electric characteristics of an apparatus, such as a semiconductor device, a liquid crystal display (LCD), a magnetic recording device, and so on. For example, electric characteristics of a semiconductor device include, but not limited to logical characteristic of a logic circuit, voltage-current characteristic, threshold voltage of MOSFET, driving current, gate leak current, hot carrier resistivity, short fault or breaking fault of interconnections, wiring resistance, and capacitance.
The silicon pin may have a metal silicide layer on the bottom face. The metal silicide facilitates plating a conductive layer on the bottom face of the silicon pin. The metal silicide has an alloy-forming temperature below an alloy-forming temperature of silicon and the conductive layer. Such a metal silicide includes, but is not limited to nickel silicide (Ni2Si), platinum silicide (PtSi, Pt2Si), and lead silicide (Pb2Si).
In another aspect of the invention, a probe assembly, which comprises a probe pin and an electrode positioned directly below and connected to the bottom of the probe pin, is provided. The probe pin comprises a silicon pin core and a conductive layer covering the entire surface of the silicon pin core. The significant feature of the probe assembly is that no leads or extracted electrodes are required.
The silicon pin core may have a metal silicide at its bottom. In this case, the metal silicide has an alloy-forming temperature below an alloy-forming temperature of the conductive layer and silicon. The electrode is connected to the bottom of the probe pin by soldering, or using a bonding agent or a resin.
In still another aspect of the invention, a probe card is provided. The probe card comprises one or more probe pins and a print wiring board having one or more electrode, each electrode being positioned below an associated probe pin, and connected to the bottom of the associated probe pin. Each probe pin comprises a silicon pin core and a conductive layer covering the entire surface of the silicon pin core. Because each probe pin is connected directly to the electrode at its bottom without requiring leads, the probe card has a fine-pitch and high-dense structure. This probe card can also eliminates variation in signal transfer due to differences in the lengths of the leads extracted from the associated pins. Accordingly, the probe card can measure a high-frequency signal in a stable manner. Since the lead or the extracted electrode, which was required in the conventional test probe, is eliminated, the contact resistance is greatly reduced, and highly precise measurement can be realized.
The prove card has an insulating layer filled in a space between the probe pins in such a manner that the tips of the probe pins project from the top surface of the insulating layer. The insulating layer makes the probe pins electrically independent from one another, and reinforces the probe card mechanically.
In still another aspect of the invention, a method for fabricating a probe pin is provided. With this method, one or more silicon pins are formed in the vertical direction by crystal growth. Then, the tip and the side face of each silicon pin are coated with a first metal. The tip and the side face of the silicon pin are collectively referred to as the first surface. Then, an insulating layer is formed around the silicon pins to mechanically fix the silicon pins. The insulating layer is formed by, for example, filling up the space between the silicon pins with an arbitrary insulating material in such a manner that the tips of the silicon pins project from the top surface of the insulating layer. Then, a second metal layer is formed on the bottom face of the insulating layer and the bottoms of the silicon pins. The bottom of the silicon pin is referred to as the second face. Then, the bulk is heated at a temperature higher than an alloy-forming temperature of silicon and the second metal and lower than an alloy-forming temperature of silicon and the first metal. After the heating step, non-reacted second metal is removed by, for example, selective etching. Finally, a third metal is deposited on the selectively etched surface.
The first metal is, for example, gold (Au). The second metal is, for example, nickel (Ni), lead (Pb), and platinum (Pt). The third metal is the same as the first metal, or alternatively, other high-conductive metals may be used. The first and second metals are selected so that the alloy-forming temperature of silicon and the first metal is higher than the alloy-forming temperature of silicon and the second metal. With this method, the bottom face of the silicon pin core is covered with a high-conductive metal, and therefore, the probe pin can be connected directly to an electrode placed in a wiring board.
An alternative method may be used to fabricate a probe pin for testing the electric characteristics of an apparatus. In the alternative method, one ore more silicon pins are formed in the vertical direction by, for example, crystal growth. Then, the first face of each silicon pin, that is, the tip and the side face of the silicon pin, is coated with a first metal. Then, an insulating layer is formed around the silicon pins to mechanically fix the silicon pins by filling up the space between the silicon pins with an arbitrary insulating material. Then, a recess is formed at the bottom (that is, the second face) of each silicon pin by, for example, etching. Finally, a third metal layer is formed so as to cover the bottom face of the insulating layer and the recesses of the silicon pins.
The third metal is then removed from the bottom face of the insulating layer, while leaving the third metal inside the recesses. The bottoms of the silicon pins covered with the third metal are connected to electrodes arranged in a print board by one-to-one correspondence.
With this alternative method, the bottom of the silicon pin is covered directly with a conductive layer without forming a metal silicide film on the bottom thereof. Accordingly, the bottom of the silicon pin coated with a conductive layer can be connected directly to an electrode.